Symmetry breaking in electron-impact dissociative ionization of linear symmetric molecules

This work provides experimental evidence of symmetry breaking in electron-impact dissociative ionization of CO2. Angular correlation between the recoil direction of fragment ions and transferred momentum is measured. It reveals that despite the inversion symmetry of the CO2 molecule, CO+ is emitted preferentially in the momentum transfer direction near the thresholds of ionization to close-lying (2)Pi(u) and (2)Pi(g) states. Similar forward-backward asymmetry is found also for dissociative ionization of N-2. The findings cannot be explained if ionization and dissociation are two independent steps, indicating that the Coulomb field of the slow ejected electron induces the mixing of gerade and ungerade states in the molecular ion.